Aftertreatment system having multiple dosing circuits

ABSTRACT

An aftertreatment system is disclosed for use with a combustion engine. The aftertreatment system may have at least one exhaust passage, and a plurality of dosing circuits configured to inject reductant into the at least one exhaust passage. The aftertreatment system may also have a controller in communication with each of the plurality of dosing circuits. The controller may be configured to determine a failure of a first of the plurality of dosing circuits, and to selectively adjust operation of a second of the plurality of dosing circuits based on the failure.

TECHNICAL FIELD

The present disclosure relates generally to an aftertreatment systemand, more particularly, to an aftertreatment system having multipledosing circuits.

BACKGROUND

Internal combustion engines, including diesel engines, gasoline engines,gaseous fuel-powered engines, and other engines known in the art exhausta complex mixture of air pollutants. These air pollutants can include,among other things, gaseous compounds such as the oxides of nitrogen(NO_(X)). Due to increased awareness of the environment, exhaustemission standards have become more stringent, and the amount of NO_(X)emitted from an engine may be regulated depending on the type of engine,size of engine, and/or class of engine. In order to ensure compliancewith the regulation of these compounds, some engine manufacturers haveimplemented a process called Selective Catalytic Reduction (SCR).

SCR is a process where a reductant (most commonly a urea/water solution)is injected into the exhaust gas stream of an engine and adsorbed onto acatalyst. The reductant reacts with NO_(X) in the exhaust gas to formwater (H₂O) and elemental nitrogen (N₂), both of which are unregulated.Care should be taken so that the amount of reductant injected into theexhaust gas stream corresponds with the amount of NO_(X) in the exhaustgas stream. If too much reductant is injected, some of the reductant maypass through the exhaust system and be discharged into the atmosphere.This can be costly and violate regulations in some areas. If too littlereductant is injected, the NO_(X) may not be adequately reduced. In somesituations, such as during a dosing circuit abnormality (e.g., duringdegraded conversion performance, during a failure, or during alow-dosing event), it can be difficult to accurately control the amountof NO_(X) being injected.

An exemplary aftertreatment system is disclosed in U.S. PatentPublication No. 2012/0204542 of Norris et al, that published on Aug. 16,2012 (“the '542 publication”). Specifically, the '542 publicationdescribes a system having two exhaust legs configured to receiveparallel flows of exhaust from an engine. A particulate filter isdisposed within each leg at a location upstream of an SCR catalyst. Ahydrocarbon closer is positioned between each particulate filter and thecorresponding SCR catalyst, and a sensor (e.g., an ammonia sensor, aNO_(X) sensor, a temperature sensor, and/or a pressure sensor) islocated downstream of each catalyst. A controller is configured todetermine clogging of the particulate filters based on signals from thesensor, calculate uneven flow distribution through the legs based on theclogging, and selectively adjust exhaust flow through the legs via athrottle to compensate for the uneven flow distribution. In addition,operation of the hydrocarbon dosers is controlled based on feedback fromthe sensor.

While the system of the '542 publication may help to maintain dosingaccuracy during uneven exhaust flow caused by particulate filterclogging, the system may still be less than optimal. For example, thesystem may not be capable of detecting or accommodating a dosing circuitabnormality.

The present disclosure is directed at overcoming one or more of theshortcomings set forth above and/or other problems of the prior art.

SUMMARY

In one aspect, the present disclosure is directed to an aftertreatmentsystem. The aftertreatment system may include at least one exhaustpassage, and a plurality of dosing circuits configured to injectreductant into the at least one exhaust passage. The aftertreatmentsystem may also include a controller in communication with each of theplurality of dosing circuits. The controller may be configured todetermine a failure of a first of the plurality of dosing circuits, andto selectively adjust operation of a second of the plurality of dosingcircuits based on the failure.

In another aspect, the present disclosure is directed to a method ofdosing reductant. The method may include dosing reductant into anexhaust flow using a plurality of dosing circuits. The method may alsoinclude determining a failure of a first of the plurality of dosingcircuits, and selectively adjusting operation of a second of theplurality of dosing circuits based on the failure.

In yet another aspect, the present disclosure is directed to an engine.The engine may include an engine block at least partially defining aplurality of combustion chambers, an exhaust manifold extending from theplurality of combustion chambers, and a turbocharger connected to theexhaust manifold. The engine may also include a first branch passageconnected to an outlet of the turbocharger, and a second branch passageconnected to the outlet of the turbocharger in parallel with the firstbranch passage. The engine may further include at least a first dosingcircuit associated with the first branch passage, at least a seconddosing circuit associated with the second branch passage, and at leastone sensor configured to generate a signal indicative of a performanceparameter of the at least a first and at least a second dosing circuits.The engine may additionally include a controller in communication withthe at least one sensor and each of the at least a first and at least asecond dosing circuits. The controller may be configured to make adetermination of one of a failure of the at least a first dosing circuitand occurrence of a low-dosing event based on the signal, to selectivelyinhibit operation of the at least a first dosing circuit based on thedetermination, and to selectively increase dosing of the at least asecond dosing circuit based on the determination.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic illustration of an engine having an exemplarydisclosed aftertreatment system.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary engine 10. For the purposes of thisdisclosure, engine 10 is depicted and described as a diesel-fueled,internal combustion engine. However, it is contemplated that engine 10may embody any other type of combustion engine such as, for example, agasoline engine or a gaseous fuel-powered engine burning compressed orliquefied natural gas, propane, or methane. Engine 10 may include anengine block 12 at least partially defining a plurality of cylinders 14,and a plurality of piston assemblies (not shown) disposed withincylinders 14 to form a plurality of combustion chambers (not shown). Itis contemplated that engine 10 may include any number of combustionchambers and that the combustion chambers may be disposed in an in-lineconfiguration (shown), in a “V” configuration, in an opposing-pistonconfiguration, or in any other conventional configuration.

Multiple separate sub-systems may be associated within engine 10 andcooperate to facilitate the production of power. For example, engine 10may include an air induction system 16, an exhaust system 18, and anaftertreatment system 20. Air induction system 16 may be configured todirect air or an air and fuel mixture into engine 10 for subsequentcombustion. Exhaust system 18 may exhaust byproducts of combustion tothe atmosphere. Aftertreatment system 20 may function to reduce thedischarge of regulated constituents produced by engine 10 to theatmosphere.

Air induction system 16 may include multiple components configured tocondition and introduce compressed air into cylinders 14. For example,air induction system 16 may include an air cooler 22 located downstreamof one or more compressors 24. Compressor(s) 24 may be connected topressurize inlet air directed through cooler 22. It is contemplated thatair induction system 16 may include different or additional componentsthan described above such as, for example, a throttle valve, variablevalve actuators associated with each cylinder 14, filtering components,compressor bypass components, and other known components that may beselectively controlled to affect an air-to-fuel ratio of engine 10, ifdesired. It is further contemplated that compressor(s) 24 and/or cooler22 may be omitted, if a naturally aspirated engine is desired.

Exhaust system 18 may include multiple components that condition anddirect exhaust from cylinders 14 to the atmosphere. For example, exhaustsystem 18 may include an exhaust manifold 26 and one or more turbines 28driven by exhaust flowing through manifold 26. Before or after reachingturbine(s) 26, exhaust manifold 26 may split into at least two parallelbranch passages 30, 32. Branch passages 30, 32 may be identical ordifferent, and lead from turbine(s) 28 to the atmosphere. It iscontemplated that exhaust system 18 may include different or additionalcomponents than described above such as, for example, bypass components,an exhaust compression or restriction brake, an attenuation device, andother known components, if desired.

Each of turbine(s) 26 may be located to receive exhaust leaving engine10, and may be connected to one or more compressors 24 of air inductionsystem 16 by way of a common shaft to form a turbocharger. As the hotexhaust gases exiting engine 10 move through turbine(s) 28 and expandagainst vanes (not shown) thereof, turbine(s) 28 may rotate and drivethe connected compressor(s) 24 to pressurize inlet air.

Aftertreatment system 20 may include components configured to trap,catalyze, reduce, or otherwise remove regulated constituents from theexhaust flow of branch passages 30 and 32 prior to discharge to theatmosphere. For example, aftertreatment system 20 may include, amongother things, a plurality of dosing circuits 34 each having one or morecatalyst substrates 36 located downstream from one or more reductantinjectors 38. In one embodiment, a single dosing circuit 34 may beassociated with each of branch passages 30, 32, in another embodiment(shown in FIG. 1), multiple dosing circuits 34 may be associated witheach of branch passages 30, 32 and disposed in series. In eitherembodiment, a gaseous or liquid reductant, most commonly urea((NH₂)₂CO), a water/urea mixture, a hydrocarbon such as diesel fuel, orammonia gas (NH₃), may be sprayed or otherwise advanced into the exhaustflow of passages 30, 32 at a location upstream of catalyst substrate(s)36 by reductant injector(s) 38. This process of injecting reductantupstream of catalyst substrate(s) 36 is known as dosing.

Catalyst substrates 38 may be arranged into bricks or packs, which areplaced in parallel and/or series relative to the flow of exhaust inbranch passages 30, 32. For example, an arrangement of multipleindividual substrates 36 may be placed to receive exhaust flow from aparticular one of branch passages 30 or 32 in parallel with each other.In this configuration, a primary exhaust flow may be divided between thedifferent substrates 36, pass through the substrates 36, and then rejoininto a single flow again at a location downstream of the substrates 36.In other configurations, the exhaust from a single branch passage 30 or32 may pass through multiple layers of substrates 36 as a single flow orthrough multiple layers of substrates 36 wherein the substrates of eachlayer are arranged in parallel with each other. Many differentconfigurations may be possible.

To facilitate dosing of catalyst substrate(s) 36 by reductant injectors38, an onboard supply 40 of reductant and a pressurizing device (e.g., apump) 42 may be associated with reductant injectors 38. In someembodiments, a single supply 40 and/or a single pump 42 may beassociated with multiple or all of injectors 38. In the disclosedembodiment, however, each injector 38 is provided with a dedicatedsupply 40 and a dedicated pump 42. Other configurations may also bepossible. The reductant sprayed into branch passages 30, 32 may flowdownstream with the exhaust from engine 10 and be adsorbed onto anupstream surface of catalyst substrate(s) 36, where the reductant mayreact with NO_(X) (NO and NO₂) in the exhaust gas to form water (H₂O)and elemental nitrogen (N₂), both of which may be unregulated. Thisprocess performed by substrate(s) 36 may be most effective when aconcentration of NO_(X) to NO₂ supplied to substrate(s) 36 is about 1:1.

To help provide the correct ratio of NO to NO₂, an oxidation catalyst 44may be located upstream of substrates 36 and injectors 38, in someembodiments. Oxidation catalyst 44 may be, for example, a dieseloxidation catalyst (DOC). As a DOC, oxidation catalyst 44 may include aporous ceramic honeycomb structure or a metal mesh substrate coated witha material, for example a precious metal, which catalyzes a chemicalreaction to alter the composition of the exhaust. For instance,oxidation catalyst 44 may include a washcoat of palladium, platinum,vanadium, or a mixture thereof that facilitates the conversion of NO toNO₂.

In one embodiment, oxidation catalyst 44 may also perform particulatetrapping functions. That is, oxidation catalyst 44 may be a catalyzedparticulate trap such as a continuously regenerating particulate trap ora catalyzed continuously regenerating particulate trap. As a particulatetrap, oxidation catalyst 44 may function to trap or collect particulatematter.

Aftertreatment system 20 may also include components configured to helpregulate the treatment of exhaust by dosing circuits 34 prior todischarge to the atmosphere. Specifically, exhaust control system 20 mayinclude a controller 46 in communication with one or more sensors 48 andwith the components of each dosing circuit 34. And based on input fromeach of sensors 48, controller 46 may determine an amount of NO_(X)being produced by engine 10, a performance parameter of catalystsubstrates 36 (e.g., a reduction efficiency), a history of theperformance parameter (e.g., the reduction efficiency tracked over aperiod of time), an amount of reductant passing through catalystsubstrates 36, a failure of any one or more of dosing circuits 34,and/or an amount of reductant that should be sprayed by reductantinjectors 38 into the exhaust flow of branch passages 30, 32 tosufficiently reduce the NO_(X) present within the exhaust in light ofcurrent conditions (e.g., in light of any known abnormalities such ascomponent failures, inefficiencies, low-dosing events, etc.). Controller46 may then regulate operation of each dosing circuit 34 to inject (orstop injecting) an appropriate amount of urea into the exhaust flows ofbranch passages 30, 32 such that an overall level of exhaustconstituents being discharged to the atmosphere by both branch passages30, 32 is less than a desired and/or regulated level.

Controller 46 may embody a single or multiple microprocessors, fieldprogrammable gate arrays (FPGAs), digital signal processors (DSPs), etc,that include a means for controlling an operation of aftertreatmentsystem 20 in response to signals received from the various sensors. Ifmultiple microprocessors are utilized, the different microprocessors maycommunicate with each other and/or with a master controller, if desired,to accomplish the disclosed functions. For example, a dedicatedmicroprocessor may be associated with each dosing circuit 34 andconfigured to control dosing of only that circuit (based on commandsfrom a common master controller), to detect abnormalities of thatcircuit, and to communicate the abnormalities to the master controller.Numerous commercially available microprocessors can be configured toperform the functions of controller 46. It should be appreciated thatcontroller 46 could readily embody a microprocessor separate from thatcontrolling other non-exhaust related engine functions, or thatcontroller 46 could be integral with a general engine microprocessor andbe capable of controlling numerous engine functions and modes ofoperation. If separate from a general engine microprocessor, controller46 may communicate with the general engine microprocessor via data linksor other methods. Various other known circuits may be associated withcontroller 46, including power supply circuitry, signal-conditioningcircuitry, actuator driver circuitry (i.e., circuitry poweringsolenoids, motors, or piezo actuators), communication circuitry, andother appropriate circuitry.

Sensors 48 may be any type of sensors known in the art that provide anindication as to functionality and/or performance of individual dosingcircuits 34. For example, a first sensor 48 a may embody a constituentsensor configured to generate a constituent signal indicative of apresence and/or concentration of a particular constituent within theexhaust flow of one or both of branch passages 30, 32 at a locationupstream and/or downstream of catalyst substrates 36. For instance,first sensor 48 a may be a NO_(X) sensor configured to determine anamount (i.e., a quantity, a relative percent, a ratio, etc.) of NOand/or NO₂ present within the exhaust of engine 10. First sensor 48 amay generate the constituent signal and send it to controller 46 forfurther processing.

A second sensor 48 b of aftertreatment system 20 may embody a reductantsensor configured to generate a slip signal indicative of a presence ofreductant within the exhaust flow of branch passages 30, 32 downstreamof catalyst substrates 36. Second sensor 48 b may generate the slipsignal and send it to controller 46 for further processing.

A third sensor 48 c of aftertreatment system 20 may be associated withsupply 40, pump 42, and/or injector 38. For example, sensor 48 c may bea fluid level sensor, a temperature sensor, and/or a pressure sensorconfigured to generate a reductant signal indicative of an amount ofreductant available (e.g., an amount of reductant remaining and/orthawed) for injection. Alternatively, third sensor 48 c could beconfigured to generate a signal indicative of a displacement position ofpump 42, a pressure of injector 38, and/or a functional status of pump42 and injector 38. This signal may be directed from sensor 48 c tocontroller 46 for further processing.

It is contemplated that sensors 48 could be fewer or greater in number,have different functionality, and/or be associated with differentcomponents of aftertreatment system 20, if desired. It is alsocontemplated that any one or more of sensors 48 may alternatively embodya virtual sensor. A virtual sensor may produce a model-driven estimatebased on one or more known or sensed operational parameters of engine 10and/or aftertreatment system 20. For example, based on a known operatingspeed, load, temperature, boost pressure, ambient conditions (humidity,pressure, temperature, etc.), and/or other parameters of engine 10, amodel may be referenced to determine an amount of NO and/or NO₂ producedby engine 10. Similarly, based on a known or estimated NO_(X) productionof engine 10, a flow rate of exhaust exiting engine 10, and/or atemperature of the exhaust, the model may be referenced to determine anamount of NO and/or NO₂ leaving catalyst 44 and entering catalystsubstrate 36. As a result, any signal (e.g., the constituent productionsignal) directed from sensor 48 to controller 46 may be based oncalculated and/or estimated values rather than direct measurements, ifdesired. It is contemplated that rather than a separate element, thesevirtual sensing functions may alternatively be accomplished bycontroller 46, if desired.

As will be described in the following section, the signals from sensors48 may be utilized by controller 46 to determine an abnormality ofaftertreatment system 20 (e.g., of a particular dosing circuit 34 ofaftertreatment system 20). And based on the abnormality, controller 46may be configured to adjust operation of aftertreatment system 20 (e.g.,to adjust operation of a different dosing circuit 34) to accommodate theabnormality and maintain engine 10 compliant with emission regulations.

INDUSTRIAL APPLICABILITY

The aftertreatment system of the present disclosure may be applicable toany engine where consistent emission control is desired. The disclosedaftertreatment system may be particularly applicable to diesel engineapplications for use in maintaining NO_(X) produced by the engine belowregulated levels, even when experiencing system abnormalities. Operationof aftertreatment system 20 will now be described in detail.

During operation of engine 10, aftertreatment system 20 may experienceany number of different abnormalities that have the potential tonegatively affect exhaust emissions. If not otherwise accounted for,these abnormalities could result in the forced shutdown of engine 10,causing a loss of productivity and stranding the associated machine awayfrom service. Examples of the different abnormalities can include systemdegradation, system failure, and low-dosing events. Each of theseexamples will be explored below to further illustrate the disclosedconcepts.

System degradation may be a normally occurring phenomenon that isexhibited by a slow reduction in NO_(X) conversion efficiency over time.In particular, over time, catalyst substrates 36 may age and lose theirability to convert NO_(X) to H₂O and N₂. This reduction in efficiencycan be exhibited by an increase in an amount of NO_(X) detecteddownstream of catalyst substrates 36 and/or an increase in an amount ofreductant injected into the exhaust at an upstream location in order tosufficiently reduce the amount of NO_(X) normally present in theexhaust. In some instances, the reduction in efficiency may be caused byonly one catalyst substrate 36 and/or one brick, of substrates 36. Thatis, the different catalyst substrates 36 of a particular aftertreatmentsystem 20 (i.e. of each dosing circuit 34) may not age at the same rate.

Many different actions may be taken in response to detecting a reductionin NO_(X) conversion efficiency of a particular catalyst substrate 36.For example, additional reductant may be injected by the dosing circuits34 associated with the underperforming catalyst substrates 36. Whilethis may be effective in some situations, in other situations anincrease in reductant injections may only serve to waste reductantwithout significantly improving NO_(X) conversion. That is, thesubstrate 36 may already be saturated and injecting additional reductionmay not be helpful. Instead, it may be more beneficial to increasereductant injections by dosing circuits 34 not associated with theunderperforming catalyst substrate 36 in an effort to offset the reducedefficiency. For example, if the catalyst substrates 36 of branch passage30 are determined to be underperforming, it may be beneficial toincreasing dosing of the catalyst substrates 36 of branch passage 32. Inthis example, although the NO_(X) conversion of branch passage 30 maynot improve, the conversion of NO_(X) in branch passage 32 may improveenough to offset the higher levels of NO_(X) being discharged frombranch passage 30. In particular, for a given engine 10, the sum ofemissions produced by all branch passages 30, 32 is what is regulatedand not the emissions of each individual branch passage 30, 32.Accordingly, if branch passage 32 is controlled to have extremely lowNO_(X) emissions, the overall NO_(X) emissions of the associated engine10 may still be less than a regulated amount, even though branch passage30 may be discharging a majority of the emissions. Accordingly, when theabnormality of degraded performance is detected with respect to oneparticular dosing circuit 34 (e.g., of a dosing circuit 34 associatedwith branch passage 30), controller 46 may adjust operation of anotherdosing circuit 34 (e.g., of a dosing circuit 34 associated with anotherbranch passage 32) to account for the degradation.

Many different types of system failures may be possible. For example, aparticular supply 40 of reductant could freeze (e.g., a heater and/ortemperature sensor associated with the supply 40 may fail), supply 40may have been drained of reductant (e.g., supply 40 may leak), pump 42may be damaged or leaking, injector 38 may stop functioning or inject anamount of reductant different from what is desired, etc. When a systemfailure occurs, reductant may not be injected at all or injected in anamount different that required to adequately reduce NO_(X) withoutwasting reductant. These failures can be detected in any number ofdifferent ways (e.g., based on NO_(X) detection, reductant detection,temperature detection, pressure detection, etc.). In addition,controller 46 may determine system failure based on signals generated bythe failed components themselves and/or based on signals generated byother engine systems. That is, it may be possible for controller 46 tonot detect the failure directly, but instead simply receive notificationof a failure.

When a component of a particular dosing circuit 34 is determined to havefailed, controller 46 may accommodate the failure using the remainingoperational dosing circuits 34. For example, if one dosing circuit 34associated with branch passage 30 is determined to have failed, theremaining dosing circuit 34 also associated with branch passage 30 maybe caused to increase its dosing to a level previously provided by bothdosing circuits 34. In this way, branch passage 30 may continue todischarge about the same amount of NO_(X). Additionally oralternatively, one or more of the dosing circuits 34 associated withbranch passage 32 may be caused by controller 46 to increase theirinjection amounts to accommodate the passage 30-failure by lowering theamount of NO_(X) discharged from branch passage 32 to an extremely lowlevel in the manner described above. In this way, while the amount ofNO_(X) discharged to the atmosphere by branch passage 30 may be higherdue to the failure, the overall discharge amount of engine 10 may remainsubstantially unchanged (i.e., aftertreatment system may continue tomaintain an overall consistent discharge of exhaust emissions).Accordingly, when the abnormality of failure is detected with respect toone particular dosing circuit 34, controller 46 may adjust operation ofanother dosing circuit 34 (e.g., of a dosing circuit 34 associated withthe same or another branch passage 30, 32) to accommodate the failure.

In some operations and/or applications (e.g., during idling and/oroperation at low load and speed), the amount of reductant that eachdosing circuit 34 is commanded to inject may be so low that injectionaccuracy is negatively affected. In particular, there may be alow-dosing limit for each dosing circuit 34, below which injectors 38cannot reliably inject reductant with a desired degree of accuracy. Ifunaccounted for, engine 10 could potentially violate regulations anddischarge more NO_(X) than desired at these times, even though injectors38 are being commanded to inject the correct amounts of reductant. Thelow-dosing event may be determined based on the amount of NO_(X)detected within branch passages 30, 32, based on the amount of reductantdetected downstream of catalyst substrates 36, and or based on amonitored speed and/or load of engine 10.

In these situations, instead of causing all dosing circuits 34 to injectthe same low levels of reductant, controller 46 may instead cause one ormore of dosing circuits 34 to stop injecting completely. In addition,controller 46 may then distribute the required amount of dosing betweenthe remaining operational dosing circuits 34. This may result in anincreased amount of reductant injected by each operational dosingcircuit 34, such that the amount of injected reductant for each circuit24 is above the low injection limit. Accordingly, when the abnormalityof low-dosing is detected, controller 46 may completely shut down orotherwise inhibit dosing of particular dosing circuits 34 andsimultaneously adjust operation of the remaining dosing circuits 34 toaccommodate the low-dosing event.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the aftertreatment system ofthe present disclosure without departing from the scope of thedisclosure. Other embodiments will be apparent to those skilled in theart from consideration of the specification and practice of theaftertreatment system disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with a truescope of the disclosure being indicated by the following claims andtheir equivalents.

What is claimed is:
 1. An aftertreatment system, comprising: at leastone exhaust passage; a plurality of dosing circuits configured to injectreductant into the at least one exhaust passage; and a controller incommunication with each of the plurality of dosing circuits, thecontroller being configured to: determine a failure of a first of theplurality of dosing circuits; and selectively adjust operation of asecond of the plurality of dosing circuits based on the failure.
 2. Theaftertreatment system of claim 1, wherein: the at least one exhaustpassage includes a first passage and a second passage in parallel withthe first passage; the first of the plurality of dosing circuits isassociated with the first passage; and the second of the plurality ofdosing circuits is associated with the second passage.
 3. Theaftertreatment system of claim 1, wherein: the at least one exhaustpassage includes a single passage; and the first of the plurality ofdosing circuits is disposed in series with the second of the pluralityof dosing circuits.
 4. The aftertreatment system of claim 1, wherein thecontroller is configured to increase dosing of the second of theplurality of dosing circuits based on the failure to maintain an overallconsistent discharge of exhaust emissions.
 5. The aftertreatment systemof claim 4, wherein the controller is further configured to shut downthe first of the plurality of dosing circuits based on the failure. 6.The aftertreatment system of claim 1, wherein the failure includes oneof a failure of a supply of reductant, a failure of a pump, or a failureof an injector.
 7. The aftertreatment system of claim 1, wherein thefailure includes a reduction in conversion efficiency of the first ofthe plurality of dosing circuits.
 8. The aftertreatment system of claim1, wherein the controller is further configured to: determine a demandfor dosing from each of the plurality of dosing circuits that is below athreshold amount; and selectively inhibit operation of the first of theplurality of dosing circuits to cause an amount of reductant injected bythe second of the plurality of dosing circuits to increase above thethreshold amount.
 9. The aftertreatment system of claim 1, furtherincluding at least one sensor configured to detect a performanceparameter of the plurality of dosing circuits, wherein the controller isconfigured to determine the failure based on signals from the at leastone sensor.
 10. The aftertreatment system of claim 9, wherein the atleast one sensor is an exhaust constituent sensor.
 11. Theaftertreatment system of claim 9, wherein the at least one sensor isslip Sensor.
 12. The aftertreatment system of claim 9, wherein the atleast one sensor is a reductant supply sensor.
 13. A method of dosingreductant, comprising: dosing reductant into an exhaust flow using aplurality of dosing circuits; determining a failure of a first of theplurality of dosing circuits; and selectively adjusting operation of asecond of the plurality of dosing circuits based on the failure.
 14. Themethod of claim 13, wherein: dosing reductant into an exhaust flowincludes dosing reductant into parallel legs of the exhaust flow; thefirst of the plurality of dosing circuits is associated with a first ofthe parallel legs; and the second of the plurality of dosing circuits isassociated with a second of the parallel Legs.
 15. The method of claim13, wherein dosing reductant into an exhaust flow includes dosingreductant into of the exhaust flow at multiple locations in series witheach other.
 16. The method of claim 13, wherein selectively adjustingoperation of the second of the plurality of dosing circuits includesselectively increasing dosing of the second of the plurality of dosingcircuits based on the failure to maintain an overall consistentdischarge of exhaust emissions.
 17. The method of claim 16, furtherincluding selectively shutting down the first of the plurality of dosingcircuits based on the failure.
 18. The method of claim 16, whereindetermining the failure includes determining the failure of one of asupply or reductant, a pump, or an injector.
 19. The method of claim 18,further including: determining a demand for dosing from each of theplurality of dosing circuits that is below a threshold amount; andselectively inhibiting operation of the first of the plurality of dosingcircuits so as to cause an amount of reductant injected by the second ofthe plurality of dosing circuits to increase above the threshold amount.20. An engine, comprising: an engine block at least partially defining aplurality of combustion chambers; an exhaust manifold extending from theplurality of combustion chambers; a turbocharger connected to theexhaust manifold; a first branch passage connected to an outlet of theturbocharger; a second branch passage connected to the outlet of theturbocharger in parallel with the first branch passage; at least a firstdosing circuit associated with the first branch passage; at least asecond dosing circuit associated with the second branch passage; atleast one sensor configured to generate a signal indicative of aperformance parameter of the at least a first and at least a seconddosing circuits; and a controller in communication with the at least onesensor and each of the at least a first and at least a second dosingcircuits, the controller being configured to: make a determination ofone of a failure of the at least a first dosing circuit and occurrenceof a low-dosing event based on the signal; selectively inhibit operationof the at least a first dosing circuit based on the determination; andselectively increase dosing of the at least a second dosing circuitbased on the determination so as to maintain an overall consistentdischarge of exhaust emissions from both of the first and second branchpassages.